Monday, April 7, 2008

Reverse Engineering

The industrial design of a TV is really interesting.

I got a chance to disassemble and reassemble an HDTV at work the other day (with some reason). It was a fun afternoon. Disassembly time: unclocked; Reassembly time: 38mins. No screws lost, no extra parts leftover.

I wrangled my way into getting a full technician's tool kit when they hired me, but the project only required a hex wrench and a Philips-head screwdriver.

When you take off the back plastic cover, there's an internal metal chassis. Good TVs have substantial internal shielding because of the FCC's electromagnetic interference (EMI) regulations. EMI happens when there is a voltage transition on a wire (ie, power or data). That transition has a corresponding current pulse, which produces a magnetic pulse that radiates outward. Hence, electro-magnetic radiation. EMI worsens when your voltage swings are large and fast. The larger the voltage transition, the more current you need to make it, the larger the magnetic pulse and the greater the EMI. The faster the voltage transition, the more current you need to make it, the larger the magnetic pulse and the greater the EMI.

The source of the EMI inside a TV is typically from the power supply (if it's a switching supply), unshielded internal cabling and poorly-laid out printed circuit boards (PCBs). Things like proper shielding, short cables, current-balanced signaling (low-voltage differential signaling, like that used in HDMI, or differential current mode logic), and proper terminations improve system EMI performance. However, high-volume manufacturers are cheap and won't use a multi-layer PCB with ground planes when a 1-layer board with jumpers will do. So, they ultimately have to make up for the cheap components with the expensive metal chassis a.k.a. Faraday

Not only do cheap TVs without shielding violate FCC standards, they also tend to have weird artifacts when the EMI from one part of the TV (say, the power supply) couples into another circuit (like the receiver) and introduces noise and offset into the signal path. This TV is build really well, though. The more sensitive high-speed digital circuitry is well-protected.

On the right in the picture way-back is the power supply and the high-voltage circuitry for driving the LCD. That's built on a single-layer board with all through-hole components and pretty huge electrolytic capacitors. That design method's been in play since the 1970s. The receiver board and input circuitry is on the left under a shield that screws to the main chassis - we know that because that's where the HDMI and cable inputs are.

Under the first shield is a second shield with a built-in heat-sink for the main video processor. Please note the secret to high-speed disassembly and reassembly: mark the various screw mountings with flag tape as you remove the screws. This is useful because manufacturers cut every corner possible, so you can't trust the assembly markings. The case (and the board) are over-designed, so the manufacturer will strip the actual number of components down to the bare minimum in order to get better profit margins. Some overseas low-cost, high-volume manufacturers are even known to take a PCB design and start removing components until the design fails -- then they put the last component removed back on. Bam! Instant cost-reduction. For this TV, you can see that all of the cabling is unshielded ribbon cable or unshielded 30AWG stranded cable and they aren't using any ferrite chokes (ie, that lump on your monitor cable that also helps improve EMI). But, like I said earlier, their metal chassis makes up for that, and, hey, the low-profile PCB connectors are nice.

I couldn't remove the heat-sink entirely due to some permanent plastic rivets, but I got it up enough to fully expose the receiver boards. There are two: one for all the digital functionality and another, (only part of it is shown) that handles all of the analog inputs and audio out. The first photo is of the digital inputs with the main processor/scalar. The second photo shows the cable tuner on the digital daughter board (in yet another shielded enclosure). The analog daughter board isn't shown, but it's the darker green board you see connected with the 3 flexible headers.

Then I did what I needed to do inside the TV, put it back together, and it still worked! The. End.

No comments:

Post a Comment